A polymer film (hereinafter referred to as film) has advantages such as excellent light transmission properties and flexibility, and is easy to make lighter and thinner. Accordingly, the film is widely used as an optical functional film. In particular, a cellulose ester film made of cellulose acylate or the like further has advantages such as toughness and low birefringence in addition to the above advantages. Therefore, the cellulose ester film is used as various kinds of optical functional films from a photographic sensitive film to a protective film for a polarizing filter and an optical compensation film as components of a liquid crystal display (LCD) whose market is increasingly expanded in recent years.
There are two main manufacturing methods of a film described above, that is, a melt extrusion method and a solution casting method. In the melt extrusion method, after polymers without a solvent are heated and melt, an extruder extrudes the polymers to manufacture a film. The melt extrusion method has advantages such as high productivity and relatively low equipment cost. In the melt extrusion method, however, it is difficult to precisely control the thickness of the film. In addition, since extrusion causes fine streaks (die lines) in the film, it is difficult to manufacture a high quality film which is available as an optical functional film. In the solution casting method, on the other hand, a solution containing polymers and a solvent is casted onto a casting support by using a casting die. A cast film formed on the casting support is hardened to have a self-supporting property and then is stripped from the casting support as a wet film. Then, the wet film is dried and wound as a film. The film manufactured by the solution casting method is superior to that manufactured by the melt extrusion method in terms of optical isotropy and uniformity in thickness and contains less foreign matter. Accordingly, the solution casting method is adopted for manufacturing the optical functional film.
In the case of manufacturing a multilayer film having a plurality of layers in a thickness direction, a casting device is used. The casting device includes a feed block and a casting die. A relatively high-viscosity polymer solution (hereinafter called high-viscosity dope) and a relatively low-viscosity polymer solution (hereinafter called low-viscosity dope) are supplied to the feed block. The supplied high-viscosity dope and the low-viscosity dope are sent to a joint portion in the feed block through a high-viscosity dope conduit and a low-viscosity dope conduit, respectively. In the joint portion, the high-viscosity dope and the low-viscosity dope are laminated to form multilayer dope. Then, the multilayer dope is sent to the casting die. A width increasing slot portion of the casting die widens the width of the multilayer dope in a direction orthogonal to a lamination direction by compression in the lamination direction. The multilayer dope, the width of which is widened, is ejected from an ejection outlet of the casting die onto a casting support as a multilayer cast film in which the high-viscosity dope and the low-viscosity dope are laminated each other in the thickness direction. After that, a multilayer film is obtained in the same manner as described above.
In the multilayer cast film, surface layers made of the low-viscosity dope and a base layer made of the high-viscosity dope are laminated in the thickness direction. The multilayer cast film may be comprised of, for example, a single low-viscosity dope layer and a single high-viscosity dope layer laminated each other, or two low-viscosity dope layers and one high-viscosity dope layer sandwiched between the low-viscosity dope layers. The high-viscosity dope is of such composition as to have optical properties need for the film. The low-viscosity dope is of such composition as to improve deterioration in surface smoothness, in stripping, and the like occurring during manufacturing, or ease handling of the film after manufacturing. Thus, it is possible to manufacture a multilayer film which has required optical properties, even thickness, and smooth surfaces.
In a conventional feed block, the length of an outlet of the high-viscosity dope was equal to the length of an outlet of the low-viscosity dope in a width direction in a joint portion. When a multilayer cast film is made by such a feed block, a part of the low-viscosity dope in the middle of the width direction flows into both widthwise ends and wraps around the high-viscosity dope. The so-called wraparound phenomenon occurs.
In the both widthwise ends of the multilayer cast film, where the wraparound phenomenon has occurred, the surface layer becomes thicker than in a middle portion. Since the low-viscosity dope contains a higher concentration of solvent and a lower concentration of polymers as compared with the high-viscosity dope, the both widthwise ends are hard to be a self-supporting state. Accordingly, when the multilayer cast film is stripped from the support, the both widthwise ends remain thereon. Once the low-viscosity dope remains, the remaining low-viscosity dope starts depositing. As a result, the film tears from its ends. Also, the widthwise ends of the multilayer cast film, where the wraparound phenomenon has occurred, contain a larger amount of solvent than the middle portion. Accordingly, bubbles tend to occur at ends in drying the whole multilayer cast film. Then, the film tears from a bubbled portion.
According to a casting device disclosed in Japanese Patent Laid-Open Publication No. 2002-221620, distribution pins having a cutout of a predetermined width are disposed in a joint portion of a feed block. Since low-viscosity dope flows into the joint portion through the cutout, the width of the low-viscosity dope becomes narrower than that of high-viscosity dope in multilayer dope and hence it is possible to prevent the wraparound phenomenon.
However, there were cases where the wraparound phenomenon occurred even if using the distribution pins described above. As a result of diligent study, the inventors have found out that controlling the depth of both widthwise ends of a cutout prevents the occurrence of the wraparound phenomenon.